Investments: - Investment in associated companies (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investment in associated companies
Investment in associated companies	$ 72,783	$ 72,010
National Rural Utilities Cooperative Finance Corporation
Investment in associated companies
Investment in associated companies	24,049	24,040
CT Parts, LLC
Investment in associated companies
Investment in associated companies	10,250	11,067
Georgia Transmission Corporation
Investment in associated companies
Investment in associated companies	27,285	25,872
Georgia System Operations Corporation
Investment in associated companies
Investment in associated companies	7,500	7,200
Other
Investment in associated companies
Investment in associated companies	$ 3,699	$ 3,831